This invention relates to fluidized catalytic cracking (FCC) units for converting heavy petroleum fractions to produce lighter fuel products. In particular it relates to a mixing device for combining a continuous liquid feed stream with hot fluidizable solid catalyst particles in a fast riser type FCC vessel.
Conversion of various petroleum fractions to more valuable products in catalytic reactors is well known in the refining industry where the use of FCC reactors is particularly advantageous for that purpose. The FCC reactor typically comprises a thermally balanced assembly of apparatus comprising the reactor vessel containing a mixture of regenerated catalyst and the feed and regenerator vessel wherein spent catalyst is regenerated. The feed is converted in the reactor vessel over the catalyst, and carbonaceous deposits simultaneously form on the catalyst, thereby deactivating it. The deactivated (spent) catalyst is removed from the reactor vessel and conducted to the regenerator vessel, wherein coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. The reactor-regenerator assembly must be maintained in steady state heat balance, so that the heat generated by burning the coke provides sufficient thermal energy for catalytic cracking in the reactor vessel. The steady state heat balance is usually achieved and maintained in FCC reactors by controlling the rate of flow of the regenerated catalyst from the regenerator to the reactor by means of an adjustable slide valve in the regenerator-to-reactor conduit.
Typically, the product stream of the catalytic cracker is fractionated into a series of products, including gas, gasoline, light gas oil, and heavy cycle gas oil. A portion of the heavy cycle gas oil is usually recycled into the reactor vessel and mixed with fresh feed. The bottom effluent of the fractionator is conventionally subjected to settling and the solid-rich portion of the settled product may be recycled to the reactor vessel in admixture with the heavy cycle gas oil and feed.
In a modern FCC reactor, the regenerated catalyst is introduced into the base of a riser reactor column in the reactor vessel. A primary purpose of the riser reactor is to crack the petroleum feed. The regenerated hot catalyst is admixed in the bottom of the riser reactor with a stream of fresh feed and recycled petroleum fractions, and the mixture is forced upwardly through the riser reactor. During the upward passage of the catalyst and of the petroleum fractions, the petroleum is cracked, and coke is simultaneously deposited on the catalyst. The coked catalyst and the cracked petroleum components are passed upwardly out of the riser and through a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked petroleum fraction is conducted to product separation, while the coked catalyst, after steam stripping, passes into the regenerator vessel and is regenerated therein, as discussed above. Most of the cracking reactions in such modern FCC units take place in the riser reactor. Accordingly, the remainder of the reactor vessel is used primarily to separate entrained catalyst particles from the petroleum fractions.
Further details of FCC processes can be found in: U.S. Pat. Nos. 3,152,065 (Sharp et al); 3,261,776 (Banman et al); 3,654,140 (Griffel et al); 3,812,029 (Snyder); 4,093,537 (Gross et al); 4,118,337 (Gross et al); 4,118,338 (Gross et al); 4,218,306 (Gross et al); 4,444,722 (Owen); 4,459,203 (Breech et al); as well as in Venuto et al, Fluid Catalytic Cracking With Zeolite Catalysts, Marcel Dekker, Inc. (1979). The entire contents of all of the above patents and publications are incorporated herein by reference.
Performance characteristics of FCC reactors can be measured by a number of factors, e.g., conversion of feed to all of the products of the FCC reactor, such as gasoline, coke and gas; selectivity of the conversion of feed to gasoline grade products; and octane number of product gasoline.
Conventional FCC catalyst may be used in the reactor utilizing the process. Suitable catalysts are, for example, those containing silica and silica alumina or mixtures thereof. Particularly useful are acidic zeolites, preferably low coke-producing crystalline zeolite cracking catalysts comprising faujasite, crystalline zeolites and other zeolites known in the art. Typically, the catalyst is a fine particle having an average size of about 20 to 100 microns.
In FCC cracking hot catalyst (650.degree. C..+-.) is mixed with relatively cold (150.degree.-375.degree. C.) charge stock. The catalyst is the heat transfer medium for vaporizing and superheating the oil vapor to a temperature suitable for the desired cracking reaction (480.degree.-545.degree. C.). In the initial stage of mixing oil and catalyst, some oil is inevitably heated to a temperature approaching that of the hot catalyst with consequent overcracking, creating a large increase in gas make. Coking of the catalyst is particularly heavy when the hot catalyst contacts oil in the liquid phase above cracking temperature.
It is an object of the present invention to control the initial mixing so as to minimize localized overheating and decrease coking.